Friday, November 16, 2012

Going extinct

kw: book reviews, nonfiction, genetics, sociology

A few weeks ago I reviewed a book by Bryan Sykes, and liked it so much I began looking for his other books. The first I encountered is Adam's Curse: A Future Without Men, published in 2003. The actual prospect of male extinction (and, probably human extinction) is taken up on the last chapter, after quite a bit of necessary ground has been covered.

The first serious matter we need to understand is the three kinds of DNA that we all contain:
  • Firstly, we all have 22 pairs of autosomes, that make up 44 of our 46 chromosomes. Almost always, we receive one set of 22 from our father and the other set of 22 from our mother. During production of the gametes (egg or sperm cells), the autosomes in a man or woman couple up and crossover, mixing the DNA from one set of grandparents so the gamete receives about an even mix of both.
  • Secondly, there are the sex chromosomes. Females have two X chromosomes, while males have one X and one Y (exceptions are rare and cause trouble). Females get one of their mother's X chromosomes (which has experienced crossover) and their father's only X chromosome (which hasn't). Males get an X from their mother, and the Y from their father.
  • Thirdly, there is a little DNA in our mitochondria, the cellular power plants that most likely started out as bacteria, but got incorporated into larger cells. This comes in the egg only, so whether we are male or female, our MtDNA is always only from our mother.
Much of Dr. Sykes's work has been using MtDNA and Y chromosomes to track population migrations around the world. He covers this work as briefly as he can (it takes up half the book). To be even more brief, let's first consider this diagram:
Joseph Macy is seven generations back in my tree, on my mother's side. I chose him because, being a Quaker, his family history is very well recorded. He was born and died on Nantucket Island (The little green leaf icons are nagging me to check out some hints they have found to add documentation to my records about all these folks).

The reason for using MtDNA and the Y are because they are so well focused. Joseph Macy had the same Y chromosome in him that was in Thomas Macy at upper right, the great-great grandfather who was born and died in England. That same Y chromosome, with perhaps a minor mutation every dozen generations, runs in the paternal line far, far back in time. He also had the same MtDNA as Katherine Reynolds, the great-great grandmother who immigrated from England with her husband Edward Starbuck. That same MtDNA, also with rare mutations, traces back through the female line, far, far back in time.

If a random man had his Y chromosome "typed", and it turned out to be the same as mine, we would have to conclude that he and I had a common ancestor, and that the descent for both of us was strictly patrilineal. If any other random person were found to have the same MtDNA as I, we are also related, but through strictly matrilineal lines. Of course, my son has his mother's MtDNA, and none of mine.

There is no such clear signal with any of the autosomes. Yes, you do have one set from your mother and one from your father, but every one of the 44 is a mosaic of pieces from two grandparents, also from four great-grandparents, and so forth. Since there are less than 24,000 genes, if you go back more than 14 generations, it is certain that for some of the ancestors in that generation, exactly none of their genes has made it to you. So there are some genetic tracks that can be followed using autosomes, but they are not nearly as well focused as the maternal and paternal tags found in the MtDNA and the Y.

Some interesting conclusions result from comparing human society in pre-agricultural times with today. In the very few non-agricultural people that still exist, property is a minor concept, limited to a few personal items a nomadic person can carry, and perhaps a tent or yurt or other portable dwelling. If one person is better off than another, it is due primarily to diligence and character. But in every other social system something else has happened. Property now mainly means ownership of land; agriculture promotes stability rather than nomadism, so possessions can be accumulated leading to Wealth; and the more complex social systems of agricultural societies need managers and overseers and leaders, leading to accumulation of Power. Property, Wealth and Power form the three-legged stool that has held most human societies together for the past 8-10 millennia.

As it happened, this became a happy hunting ground for the Y chromosome. Where pre-agricultural societies are egalitarian or matriarchal, agricultural societies are patriarchal. The physical strength of males led to a dominance game, driven by the Y chromosome's "need" to propagate. One matter brought out very clearly by the author is that the two kinds of non-autosomal DNA have opposed interests. MtDNA doesn't "care" about males. The sperm carries no MtDNA into the next generation, only the egg. The Y chromosome doesn't "care" about females, which never have one (or if one does, by some fluke, it isn't passed on anyway).

If you only look at the nuclear DNA, there is an imbalance. Men make tens of millions of new sperm cells every day. A woman is born with a thousand or so eggs, and if she never mates, releases one (or rarely two) monthly for 30-35 years, thus using about 400. If she bears a child every two years, as many women did in prior generations, and ovulates no more than 2-3 times between pregnancies, AND lives through the process, by age 55 she will have released 40-50 eggs, no more. However, when you look at MtDNA, there are tens of thousands of mitochondria in each egg cell, which rebalances the scales a bit. Also, while a woman's mitochondria go to all her children, they only get to her grandchildren through her daughters. Her sons are mitochondrial dead ends.

The remaining imbalance has led to opposed strategies for getting one's DNA propagated. Women want lots of daughters to pass on both their nuclear DNA and their MtDNA. Men want lots of sons to pass on their Y chromosome. Men can father lots of children, if they can persuade (or coerce) lots of women to mate. Women can mother fewer than 20 children (unless one can have lots of twins), so their strategy is more conservative. In the end, usually, women choose their mates. And in most societies, women have long known how to induce miscarriage if impregnated by the "wrong" man. As in peacocks and elephant seals and nearly all animals, sexual selection results. In humans, this leads to male greed: get more stuff means get more mates.

Property, wealth and power turn out to be strong signals of apparent fitness; at least that is how many women think. Thus it is no surprise that leading men of every society are able to gather lots of women, sometimes as explicit harems (like the King of Siam in the movie The King and I), sometimes through "serial polygamy" (like Donald Trump or any number of other celebrated and oft-married men). The inherent greed this leads to is what Dr. Sykes calls Adam's Curse. It has led to CEO's with near-billion-dollar "compensation" (often for ruining rather than running the company); and to short-term-focused "business" that has fished out the seas, filled the earth with pollution (there is DDT atop Mt. Everest), and is presently doubling down on the production of energy and its side product, carbon dioxide (This may not extinguish human life, but it will certainly lead to coastal depopulation as the seacoasts rise to a new contour). And of course, there is war. Just wait'll the water wars begin, within the next 10-20 years…

Nature has a trick or two up her sleeve, though. The Y chromosome is eroding. All the autosomes that go into gametes get "cleaned up" of most mutations during the crossover process. The Y does not (It was not emphasized that the two X's in a woman can and do cross over, so on average, an X chromosome gets detoxified every couple of generations). The Y chromosome has its own DNA repair mechanism, but it is much less effective than the machinery used during crossover. Also, MtDNA is so much more sparse and efficient, that most mutations are fatal, and do not get into viable eggs, plus a mitochondrion also has its own repair kit. So the good old Y is at risk.

What is the risk? Fertility is dropping, worldwide. Some of this is due to the chemicals we have invented since the 1880s. But some is because key genes in the Y are getting wiped out by mutations. In every generation, about one man in a thousand is sterile because of such mutations. In the last couple of chapters the author describes his reasoning and assumptions. Bottom line: The Y chromosome's continued erosion will lead to a further 99% decrease in fertility in the next 5,000 generations, or about 125,000 years (based on a 25 year generation; based on 32 year generations, it could be 160,000 years). That's a long time, but I suspect most people would have thought that if we avoid blowing ourselves up with H-bombs or polluting ourselves out of existence, we could go on for millions of years.

How much longer might it be until no fertile men could be found? He doesn't say. But he does speculate that women may find a technological way to fuse two eggs to produce a viable fetus, and thus bear children without men being involved. Such children would always be female. Men would go extinct. If such a process turns out to be impossible, then, sad to say, the women will go extinct along with the men, leaving the planet to other species.

But...with men out of the way, would women be less inclined to accumulate property, wealth and power? It is hard to say. I know women who are just as acquisitive as any greedy man (think Imelda Marcos). It could be that "Adam's Curse" is in actuality only slightly worse than the general "human curse".

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